Many authors say it's the fact, that almost any culture around the world had built them. In the 19th century, before the invention of scientific dating methods, it was thought therefore, that this was a sign of a seed culture which brought the style around the world. The idea of Atlantis was near. This idea of hyper-diffusionism (that each and every ancient invention spread from one point of origin) is lon dead in academic circles, because after introduction of dating methods it was clear, that the time span between the oldest and the youngest pyramid building culture was too large (almost 3000 years), and that building style and function of those pyramids were far to different for coming from one source.
But the Egyptian pyramids alone have enough mysteries to fill libraries with books about them. And the most mysterious pyramids are of course those at Giza, a suburb of the metropolis Cairo. And every year new books about those "mysteries" are added. Even serious books are riding on the mystery-train, so is the German title of Mark Lehners "The complete pyramids" "The Secret of the pyramids" - sounds a bit like Mendelssohns Riddle of the Pyramids.